Ion Heat Pulse Propagation Experiments at W7-X Using High Speed Charge Exchange Spectroscopy

Ion temperature clamping during ECRH in the W7-X stellarator experiment is found consistently across magnetic configurations and significantly limits device performance [1]. First results show that the clamping can in part be explained by turbulent transport driven by Ion Temperature Gradient (ITG) modes, which provide a limit on the inversed gradient length a/L_Ti. However, detailed measurements of the corresponding temperature profile stiffness via power balance analysis methods were obscured by the strong dependence on the T_e/T_i ratio, and electron-ion channel coupling. Here, we report on first ion heat pulse propagation studies, which aim to quantify the T_i profile stiffness in W7-X similar to electron heat pulse propagation experiments [2]. Successful experiments have been performed using both NBI and ECRH modulation, and clear modulation-induced fluctuations were observed in the electron and ion temperatures. For the ion-temperature measurement, a new charge exchange diagnostic has been employed, which allows for measurements with framerates >1kHz. These speeds allow for detailed comparisons of fluctuation amplitude and channel phase delay profiles with physics models for ion heat transport. A new method for forward modelling of heat transport using a numerical scheme similar to that implemented in the STRAHL code will be presented.

[1] Beurskens, M.N.A., et al. “Ion Temperature Clamping ...” Nucl. Fusion, 2021

[2] Weir, G.M., et al. “Heat Pulse Propagation …” Nucl. Fusion, 2021